Diversity antenna system



Filed Aug. 31, 1938 MW MLQQ w M l 01 Z Z m5 3 attorney Patented F e5. 6, 1940 UNITED STTES DIVERSITY ANTENNA SYSTEM Winfield R. Koch, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 31, 1938, Serial No. 227,790 r 2 Claims.

. My invention relates to a method of and means for reducing fading in radio reception, and more particularly to an electronic switching means by which a pair of spaced antennas are alternately utilized at a receiving or transmitting station.

It is well known that radio reception is subject to periodic fading, especially at high frequencies. It is also known that the signal strength of a given station may vary widely between antennas located at different points which are relatively close together. It is impractical to connect several antennas located in different positions to one receiver, because the currents in one are frequently out of phase with the currents in the other. The result is that the signal at the re ceiver is actually decreased and the fading made worse.

One solution of this problem involves the use of several receivers, each having a separate antenna, and means for automatically bringing the receiver into operation which is providing the best signal. This system, known as a diversity reception system, involves more apparatus than the usual broadcast listener can afford.

I propose to provide an effective diversity reception system which uses but one receiver, and yet which is automatically receptive to signals from the one of two antennas which provides thebest signal strength.

Alternately I propose to apply the same system to the transmitter in such a manner that fading is reduced for receivers having but one antenna.

It is an object of my invention to provide an automatically operated device for reducing fading in radio reception.

It is a further object of my invention to provide means for utilizing more than one antenna with a radio receiver to reduce fading.

A further object of my invention is to provide 1 a simple and inexpensive device for obtaining the benefits of diversity reception.

A still further object of my invention is to reduce fading at a radio receiver by transmitting alternately from two antennas which are at diiferent locations.

A still further object of my invention is to connect alternately two antennas to a transmitter or receiver at a supersonic rate.

A still further object is to provide an electronic switching means to accomplish the above-named objects.

My invention will be better understood from the following description when considered in connection with the accompanying drawing. Its scope is indicated by the appended claims. 7

Referring to the drawing: Figure 1 is a schematic diagram of one embodiment of my invention as applied to a radio receiver, and

Figure 2 is a schematic diagram of an alternative embodiment of my invention as applied to a radio transmitter.

Referring to Fig. 1, a pair of antennas 3 and 5 are provided which are located at suitably remote locations, and which are preferably separated by v a distance which is greater than one wave length at the lowest desired operating frequency. The

antennas are respectively connected to ground through the primaries l and 9 of a pair of input transformers II and I3. Input transformers H and i3 are similar to those commonly used in radio receivers. Their secondary windings i5 and I"! are respectively shunt tuned by a pair of variable capacitors I9 and 2|, which may be ganged together for unitary operation.

One terminal of each secondary windings i5, 5 "i is connected to respective by-pass capacitors 23 and 25, and also through respective resistors 35, 31 to an AVC voltage source will'be subsequently described. The remaining terminal of each secondary winding l5, I7 is connected to the respective capacitors I 9 and 2H and to the control grids 21 and 29 of a pair of thermionic tubes 3! and 32?. Tubes 31 and 33 are preferably multi-grid tubes such as are-commonly used as the first detectoroscillator in a superheterodyne receiver.

The anode electrodes 39.and M are connected together and 'to one terminal of the primary to of a coupling transformer 43. The remaining terminal of the primary 45 is connected to any convenient source of high voltage D. C. 49. Each tube is provided with a screen grid electrode 5 l, 53 which is likewise connected to the positive potential source 49. The cathode 52, 54 may be grounded directly, or through a common bias resistor-capacitor combination 55, 51.

The secondary 41 is connected to the first detector or radio frequency amplifier of a standard radio receiver 59, which is not shown in detail. The capacitors I9, 2! are preferably ganged together with the amplifier and oscillator tuning capacitors of the receiver.

It is to be noted that the circuit which has been described includes a pair of radio frequency amplifiers fed from apair of antennas and connected to a single radio receiver. The amplifiers may be an addition to the standard receiver, or may be substituted for the usual first stage of radio frequency amplification. It is evident, however, that the device would be inoperative unless the currents from the two antennas were in'proper phase, or unless some means were provided for making one-half of the system inoperative. Manual selection is undesirable and therefore I propose to accomplish the desired. result by making the tubes 3I, 33 alternately non-conductive at a rate which is high enough so as not to cause any interruption in the signal reception. The first and second grids BI, 63 and 55, 61 of the respective tubes are therefore interconnected and constitute an impulse oscillator which alternately biases off the tubes.

A pair of grid resistors 39 and H are respectively connected between grids GI and 85 and the cathodes 52, 54. A pair ofresistors'13, are respectively connected between grids 63 and '91 and a tap on the positive potential power supply 49. A coupling capacitor 11 is connected from grid GI to grid 51, and a similar coupling capacitor 19 is connected between grid 65 and grid 63.

The values of the resistors and capacitors which constitute this oscillator are suitably selected so that the resultant impulse oscillation is of the desired supersonic frequency. In such a case the receiver 59 will effectively be connected first to one antenna and then to the other. the signal from one antenna fades out, and the signal from the other antenna is still usable, the usable signal will operate the receiver. The possibility of simultaneous fading of the signal from both antennas is reduced by placing the antennas at different locations whichare some distance apart.

The two tubes 3I and 33 are automatically biased according to the signal having the greatest intensity by applying the receiver AVC voltage to the control grids 21' and 29. It is to be noted that this AVC voltage is applied to both tubes.

Consequently, when the signal fades out from the first antenna, there will be enough time lag in the AVG voltage to prevent the first amplifier from amplifying static and other noises at maximum amplification. The overall receiver sensitivity is thus automatically adjusted by the signal having the greatest intensity.

The system described above can be applied to a standard receiver as additional apparatus, or can be incorporated within the receiver itself. In either case a minimum of parts is required.

This system is particularly useful in conjunction with ultra high frequency receivers on police cars and the like. The fading effects at ultra high frequencies are intensified, probably due to shadows cast by buildings in certain localities, and to cancellation of one wave by a second wave travelling a different and longer path. Frequently, a change in the position of the antenna of only a few feet is sufficient to restore reception to normal. By equipping police cars with two antennas and an electronic switching system of the type herein disclosed, greatly improved reception can be obtained.

It is possible to obtain effectively the same result without modifying the receiver or adding anything to it. This is accomplished by applying the principle of my invention to the transmitter insead of to the receiver. A device of this nature is shown in Fig. 2. I

Referring to Fig. 2, a pair of dipole antennas I HI and I93, or the like, are provided. These antennas are placed at different locations and are respectively coupled through coupling transformers I35, 191 and transmission lines I99, III, to a pair of output transformers H3 and. H5.

The

output transformers are respectively connected in the anode circuits of a pair of radio frequency power amplifiers H1 and H9.

Potential for the screen grids I2I and I23 is respectively obtained from a suitable tap on two batteries I25 and I21, or the like. The negative terminals of the batteries are connected to cathodes I29, I 3| and grounded. The control grids I33, I of the power tubes are connected in parallel and energized from' a source of modulated, or unmodulated, radio frequency energy which is indicated by reference numeral I31.

The-transmitting system thus described includes a transmitter which has two output systems connected in parallel, driving separate antennas. It would be unsatisfactory to operate in this manner, however, as in certain locations the signals from the two antennas Would be out of phase and reception would be impossible. As before, therefore, an electronic commutator is utilized to cause the output tubes to become alternately operative at a supersonic rate. If at any given point reception from one antenna is poor, the signal from the other antenna may be usable.

Switching is accomplished by interconnecting a pair of triodes I39 and MI so as to obtain impulse oscillation and alternately biasing off one of the output tubes. The output amplifiers H1 and I I9 are respectively provided with suppressor grids I43 and I45. These grids are respectively connected to the anodes I41, I49 of the triodes through coupling capacitors I5I and I53. Grids I43 and I are also respectively grounded through resistors I and I51.

The grid I59 of triode I39 is grounded through a resistor I6! and coupled by a capacitor I63 to the anode I49 of triode MI. The grid I65 of triode MI is similarly grounded through resistor I61 and coupled to the anode I41 of triode I39 by capacitor I69. Anode potential is supplied by a battery I1 I, or the like, which is respectively connected to the anodes I41 and I49 through resistors I13 and I15.

The operation of this device may be explained as follows: As the triodes oscillate in the well known manner, a biasing potential is alternately impressed on the grids I43 and I45 by capacitors I5I and I53 due to the decrease in anode potential as each tube becomes conductive. Consequently the radio frequency output is transferred from one antenna to the other at a rate which is determined by the frequency of oscillation of the two triodes. As before this is preferably at a supersonic frequency.

I have thus described a system of diversity reception which may be applied either to a receiver or to a transmitter.

I claim as my invention:

1. In a device of the character described, a pair of antennas positioned in spaced relationship, a pair of thermionic tubes having input and output circuits, means respectively coupling said antennas to said input circuits, a utilization device, means coupling said utilization device to said output circuits, means for simultaneously limiting the sensitivity of both of said thermionic tubes in proportion to the signal intensity from one of said antennas and means for causing said thermionic tubes to become alternately conductive, whereby currents from said antennas are alternately impressed on said utilization device.

2. In a device of the character described, a pair of antennas positioned inspaced relationship, a pair of thermionic tubes having input, output, and auxiliary electrodes, means respectively coupling said antennas to said input electrodes, a utilization device, means coupling said utilization device to said output electrodes, means including said auxiliary electrodes for generating supersonic oscillatory currents which cause said tubes to become alternatively conductive, and means for simultaneously and equally limiting the sensitivity of said thermionic tubes in proportion to the amplitude of the received signal having the greatest intensity.

WINFIELD R. KOCH. 

